Jitter for a PPE system refers to variations in the time interval between a master timing trigger pulse applied to the input of the PPE modulator and the output pulse from the PPE modulator. The primary source of jitter in the PPE modulator is due to imperfect dc-voltage regulation of a switching power supply which supplies electrical energy to the PPE modulator. Variations in the output voltage of the switching power supply produce corresponding voltage variations in the V.sub.co voltage on the energy storage capacitor C.sub.o. These voltage variations on C.sub.o produce variations in the time delay through the magnetic assist and magnetic switches present in the magnetic pulse compression circuitry of the PPE modulator.
The time delay through a magnetic assist is given as: EQU T.sub.d =V.sub.nom T.sub.dnom V.sub.op, (1)
Where T.sub.d represents the time delay through the magnetic assist. For a magnetic assist, the term V.sub.nom T.sub.dnom represents the volt-second product of the magnetic assist, which is a constant for a given design. V.sub.op is the nominal charge voltage on the capacitor C.sub.o.
For magnetic switches, equation (1) is also used to calculate the time delays of the magnetic switches by replacing V.sub.op by the average voltage applied to the particular magnetic compression stage during its "charge time".
The delay time is inversely proportional to V.sub.op for either case.
The traditional approach to jitter compensation for a PPE system is to derive the electronic time delay required to cancel the magnetic time delay by comparing a signal proportional to V.sub.co to a linear voltage ramp. Changes in V.sub.co will produce changes in the amount of time it takes for the linear voltage ramp to reach the magnitude of the voltage signal proportional to V.sub.co. If the correct slope is chosen for the linear voltage ramp, the variation in time delay in the jitter compensation circuit will cancel the variation in time delay in the PPE modulator magnetic compression circuits due to variations in V.sub.co.
The traditional approach requires a linear voltage ramp that produces an electronic time delay in the jitter compensation circuit where the electronic time delay is exactly equal to the magnetic time delay through the PPE modulator for a given charge voltage. This is expressed in the Equation (2) below: EQU T.sub.de =T.sub.dm, (2)
Where T.sub.de and T.sub.dm, respectively, represent the time delays through the jitter compensation and magnetic compression circuits. This method represents a "global" approach to jitter compensation because the time delay in the jitter compensation circuit must always equal the magnetic time delay through the PPE modulator. A modulator having a time delay of 9.5 ms for full power operation would require a jitter compensation circuit with a 9.5 ms delay. To achieve an overall jitter requirement of 2 ns peak to peak, this compensation approach would require a high speed, high voltage (50 V input) comparator.
The nature of the charge voltage signal (V.sub.co) for a modulator driving a load device, such as a high power copper vapor laser, constrains a jitter compensation circuitry. This is due to the relatively small amount of voltage variation on the V.sub.co peak charge voltage and large volt second product of the magnetic modulator. Since it is the voltage variation that is of interest, the 5-6 volt peak to peak voltage variation riding on 950-1000 volt signal gives a poor total-signal to noise ratio. Small voltage variations produce a large amount of jitter, or temporal variation, in the modulator output pulse. The traditional method of jitter compensation described above requires a voltage ramp that is proportional to the voltage variation on the peak value of V.sub.co divided by the temporal variation (jitter) it produces. This method of jitter compensation requires a relatively slow voltage ramp since the voltage variation is small and the amount of jitter is large. Tests of high speed comparators indicates that a voltage ramp of greater than 6 VHD .mu.s is required to reduce comparator jitter to an acceptable level.
To meet the requirements of 2 ns peak to peak jitter on the output pulse of a high power magnetic modulator, a need exists for an improved technique for jitter compensation.